He said: "Everything else must be tried first before we consider taking life to save life.

"However, if we get to the point of no return as it were there's a moral duty, if not possibly a legal duty, on us as police officers who are armed and able to perhaps deflect an attack to take life to save life."

'Serious blow'

Mr Cook said the death of Mr Menezes had been a "serious blow" to relations with Brazil and police would have to look again at the policy.

He said: "They plainly will not want to have anything approaching a repeat of that and I am quite sure they will be tightening up the guidelines and the rules of engagement to make sure it doesn't happen again."

Current Met Police Commissioner Sir Ian Blair has apologised to Mr de Menezes' family and spoken of his "deep regrets", but defended the actions of his officers.

"What we have to recognise is that people are taking incredibly difficult fast time decisions in life threatening situations," he said.

Sir Ian said there was "no point" shooting a suspect in the chest as that is where a bomb would most likely be and it would detonate.

Lord Stevens told the News of the World the policy, which he described as a "shoot-to-kill-to-protect", was correct despite the chance "tragically of error".

The former Met Police Commissioner had sent teams to Israel and other countries hit by suicide bombers.

'Unique evil'

There they learned a "terrible truth" that the only way to stop a suicide bomber was to "destroy his brain instantly, utterly" to prevent a dying bomber activating a device.

Previously, officers fired at the offender's body, "usually two shots, to disable and overwhelm".

Lord Stevens said: "We are living in unique times of unique evil, at war with an enemy of unspeakable brutality, and I have no doubt that now, more than ever, the principle is right despite the chance, tragically, of error.

"And it would be a huge mistake for anyone to even consider rescinding it."